Autumn snowfall and hydroclimatic variability during the past millennium inferred from the varved sediments of meromictic Lake A, northern Ellesmere Island, Canada

We examined the hydroclimatic signal in a record of annual lamina (varve) thickness from High Arctic Lake A, Ellesmere Island (83°00.00′N, 75°30.00′W). In this unglacierized catchment, nival melt is the dominant source for meltwater and transport of sediment to the lake, and autumn snowfall is highly influential on varve thickness through the amount of snow available for melt in the following year. For the period during which climatic data are available, varve thickness in Lake A was significantly correlated (r = 0.50, p < 0.01) with the cumulative snowfall from August to October (ASO) during the previous year and, to a lesser extent, ASO mean daily temperature (r = 0.39, p < 0.01) at Alert, Nunavut (175 km east). The varve thickness record, interpreted as a proxy record of ASO snowfall and, by extension, ASO temperature, indicated above-mean conditions during five periods of the past millennium, including most of the 20th century. These results corresponded well to other available high-resolution proxy climate records from the region, with some discrepancies prior to AD 1500 and during the period AD 1700-1900.     

Global sediment core-top calibration of the TEX86 paleothermometer in the ocean

The TEX₈₆ (TetraEther indeX of tetraethers consisting of 86 carbon atoms) paleothermometer is based on the relative distribution of archaeal lipids, i.e. isoprenoid glycerol dibiphytanyl glycerol tetraethers (GDGTs), and is increasingly used to reconstruct past sea water temperatures. To establish a more extensive, global calibration of the TEX₈₆ paleothermometer, we analyzed GDGTs in 287 (in comparison with 44 in currently used calibration) core-top sediments distributed over the world oceans and deposited at different depths. Comparisons of TEX₈₆ data with (depth-weighted) annual mean temperatures of the overlying waters between 0 m and 4000 m as well as with different seasonal mean temperatures at 0 m water depth showed that the TEX₈₆ proxy reflects mostly annual mean temperatures of the upper mixed layer. The relationship between TEX₈₆ values and sea-surface temperatures (SSTs) was non-linear mainly because below 5 °C the change in TEX₈₆ values was minor with temperature. This suggests that the TEX₈₆ proxy might not be directly applicable for the Polar Oceans. Nevertheless, between 5 °C and 30 °C, the TEX₈₆ proxy has a strong linear relationship with SSTs. Here, we, therefore, propose a new linear calibration model (T = −10.78 + 56.2 ∗ TEX₈₆, r² = 0.935, n = 223) for past SST reconstructions using the TEX₈₆ palaeothermometer.     

D/H variation in terrestrial lipids from Santa Barbara Basin over the past 1400 years: A preliminary assessment of paleoclimatic relevance

We analyzed D/H ratios of common terrestrial leaf wax lipids in a 1400 year sediment core from the Santa Barbara Basin (SBB) to test whether they accurately record terrestrial climate in Southern California. The D/H ratios of long chain n-alkanes vary substantially with depth, but are poorly correlated with other terrestrial climate proxies. Interference from fossil hydrocarbons may be at least partly responsible. Long chain n-alkanoic acids exhibit nearly constant downcore D/H ratio values. This constancy in the face of known climatic shifts presumably reflects a substantial residence time for leaf wax compounds in terrestrial soil and/or on the basin flanks. Alternatively, the isotopic composition of meteoric waters in Southern California may not covary with climate, particularly aridity. However, the δD values of n-C₂₂ and n-C₂₄ fatty acids, commonly attributed to terrestrial aquatic sources, are partially correlated with Southern California winter Palmer Drought Severity Index, a tree ring-based climatic proxy (R² 0.25; p < 0.01) on multi-centennial scales with an inferred ca. 215 year time lag. The improved correlation of these biomarkers can be explained by the fact that they are not stored in terrestrial soil nor are subject to interference from fossil hydrocarbons. Our study indicates that the SBB is unlikely to preserve high resolution leaf wax D/H records that can serve as quantitative paleoclimate proxies, though some qualitative information may be retained. More generally, the sources of lipids in marginal marine basins need to be carefully evaluated prior to attempting paleoclimate reconstruction based on the leaf wax D/H proxy.     

Holocene temperature fluctuations in the northern Tibetan Plateau

Arid Central Asia (ACA) lies on a major climatic boundary between the mid-latitude westerlies and the northwestern limit of the Asian summer monsoon, yet only a few high-quality reconstructions exist for its climate history. Here we calibrate a new organic geochemical proxy for lake temperature, and present a 45-yr-resolution temperature record from Hurleg Lake at the eastern margin of the ACA in the northern Tibetan Plateau. Combination with other proxy data from the same samples reveals a distinct warm–dry climate association throughout the record, which contrasts with the warm–wet association found in the Asian monsoon region. This indicates that the climatic boundary between the westerly and the monsoon regimes has remained roughly in the same place throughout the Holocene, at least near our study site. Six millennial-scale cold events are found within the past 9000 yr, which approximately coincide with previously documented events of northern high-latitude cooling and tropical drought. This suggests a connection between the North Atlantic and tropical monsoon climate systems, via the westerly circulation. Finally, we also observe an increase in regional climate variability after the mid-Holocene, which we relate to changes in vegetation (forest) cover in the monsoon region through a land-surface albedo feedback.     

Weakening climatic signal since mid-20th century in European larch tree-ring chronologies at different altitudes from the Adamello-Presanella Massif (Italian Alps)

Tree rings from temperature-limited environments are highly sensitive climate proxies, widely used to reconstruct past climate parameters for periods prior to the availability of instrumental data and to analyse the effect of recent global warming on tree growth. An analysis of the climatic signal in five high-elevation tree-ring width chronologies of European larch (Larix decidua Mill.) from the tops of five different glacial valleys in the Italian Central Alps revealed that they contain a strong summer-temperature signal and that tree-ring growth is especially influenced by June temperatures. However, a moving correlation function analysis revealed a recent loss of the June temperature signal in the tree-ring chronologies. This signal reduction primarily involves the two lowest-altitude chronologies. It is probable that the observed increasing importance of late-summer temperature for tree-ring growth over the past 50 yr is an effect of the lengthening growing season and of the variations in the climate/tree-ring relationship over time. All the chronologies considered, especially those at the highest altitudes, show an increasing negative influence of June precipitation on tree-ring growth. The climatic signal recorded in tree-ring chronologies from the Italian Central Alps varies over time and is also differentially influenced by climatic parameters according to site elevation.     

Stable isotopic compositions of waters in the karst environments of China: Climatic implications

A total of 117 water samples, including cave water, ground water, spring water and river water, collected from the monsoonal area of China have been analyzed for their H- and O-isotope composition. Overall, a δ¹⁸O–δD correlation is observed of δD = −4.45 + 6.6δ¹⁸O (R² = 0.90) and a significant evaporation effect observed for the southern sites. Average δ¹⁸O and δD site values generally correspond to those of precipitation in nearby cities, with correlations of δD = 2.18 + 7.23δ¹⁸O (R² = 0.95) for the sample sites and δD = 11.05 + 7.95δ¹⁸O (R² = 0.95) for the cities. The effects of rainfall amount and temperature on precipitation δ¹⁸O were calculated using a simplified theoretical model derived from the Rayleigh distillation equation, which demonstrated that the sign of δ¹⁸O_pvs. T correlation is dependent on precipitation intensity. The mean δ¹⁸O value of cave waters exhibit decreasing trends with increasing latitude and reveal a spatial pattern of positive correlation with annual mean temperature and precipitation, mainly reflecting isotopic fractionations in the moisture source traveling from the ocean side to the inland continent. This spatial pattern implies that the δ¹⁸O values recorded in the proxy climate records derived from speleothems might be influenced by shifts in monsoon boundary during the past, especially between glacial and interglacial intervals.     

Tropical Atlantic SST history inferred from Ca isotope thermometry over the last 140ka

Exploring the potentials of new methods in palaeothermometry is essential to improve our understanding of past climate change. Here, we present a refinement of the published δ^44/40Ca-temperature calibration investigating modern specimens of planktonic foraminifera Globigerinoides sacculifer and apply this to sea surface temperature (SST) reconstructions over the last two glacial-interglacial cycles. Reproduced measurements of modern G. sacculifer collected from surface waters describe a linear relationship for the investigated temperature range (19.0-28.5 °C): δ^44/40Ca [‰] = 0.22 (±0.05)∗SST [°C] −4.88. Thus a change of δ^44/40Ca[‰] of 0.22 (±0.05) corresponds to a relative change of 1 °C. The refined δ^44/40Ca_modern-calibration allows the determination of both relative temperature changes and absolute temperatures in the past. This δ^44/40Ca_modern-calibration for G. sacculifer has been applied to the tropical East Atlantic sediment core GeoB1112 for which other SST proxy data are available. Comparison of the different data sets gives no indication for significant secondary overprinting of the δ^44/40Ca signal. Long-term trends in reconstructed SST correlate strongly with temperature records derived from oxygen isotopes and Mg/Ca ratios supporting the methods validity. The observed change of SST of approximately 3 °C at the Holocene-last glacial maximum transition reveals additional evidence for the important role of the tropical Atlantic in triggering global climate change, based on a new independent palaeothermometer.     

Deep-sea coral geochemistry: Implication for the vital effect

Deep-sea corals hold a great potential as a key to important aspects of paleoceanography for at least two reasons, 1) they offer temporal high resolution records of deep-sea environment, because they have growth banding structures, 2) and they are well suited for studying vital effects, because the deep-sea environment does not change over short time scales. However, the relationship between the chemical composition of deep-sea coral skeletons and environmental factors is not well understood. In this study, the chemical composition of deep-sea corals was measured in bulk individuals and along skeletal micro-structures. Among the bulk individuals, δ¹⁸O value and Sr / Ca ratio show a negative but weak correlation with ambient temperature. On the other hand, the Mg / Ca ratio has a positive, weak correlation with the temperature. Large variations were found among samples collected from similar temperature. The variation is up to 3.8‰ for δ¹⁸O, 0.9 mmol/mol for Sr / Ca ratios, and 0.78 mmol/mol for Mg / Ca ratios among samples with ambient average temperature within 1 °C. This variation may be due to a large vital effect. The centers of calcification (COCs), which were formed at high calcification rate, have lower Sr / Ca, U / Ca and higher Mg / Ca ratios than surrounding fasciculi. This chemical distribution supports the model that elemental incorporation depends on calcification rate. This suggests that calcification rate is a very important factor for the chemical composition in deep-sea corals and is one of the most significant mechanisms of the vital effect. Because of this large vital effect, further investigations are essential to use the deep-sea coral as a temperature proxy.     

North Atlantic climate variability since AD 1350 recorded in δ 18O and skeletal density of Bermuda corals

The reconstruction of the climatic history during the past several hundred years requires a sufficient geographical coverage of combined climate proxy series. Especially in order to identify causal connections between the atmosphere and the ocean, inclusion of marine records into composite climate time series is of fundamental importance. We present two skeletal δ ¹⁸O chronologies of coral skeletons of Diploria labyrinthiformis from Bermuda fore-reef sites covering periods in the nineteenth and twentieth centuries and compare them with instrumental temperature data. Both time series are demonstrated to display sea-surface temperature (SST) variability on inter-annual to decadal time scales. On the basis of a specific modern δ ¹⁸O vs instrumental SST calibration we reconstruct a time series of SST anomalies between AD 1350 and 1630 covering periods during the Little Ice Age. The application of the coral δ ¹⁸O vs temperature relationship leads to estimates of past SST variability which are comparable to the magnitude of modern variations. Parallel to δ ¹⁸O chronologies we present time series of skeletal bulk density. Coral δ ¹⁸O and skeletal density reveal a strong similarity during Little Ice Age, confirming the reliability of both proxy climate indicators. The past coral records, presented in this study, share features with a previously published climate proxy record from Bermuda and a composite time series of reconstructed Northern Hemisphere summer temperatures. The coral proxy data presented here represent a valuable contribution to elucidate northern Atlantic subtropical climate variation during the past several centuries. Received: 9 November 1998 / Accepted: 13 September 1999     

Environmental controls on branched tetraether lipid distributions in tropical East African lake sediments

Quantifying past continental temperature changes is an important aspect of paleoclimate research as it allows us to constrain the amplitude of natural variability, test predictive climate models, and provide a proper context for changes that may arise in response to anthropogenically-induced climate change. The recently developed biomarker-based methylation index of branched tetraethers/cyclization ratio of branched tetraethers (MBT/CBT) proxy shows potential as a new method for continental temperature reconstruction, but thus far it has only been applied successfully in ocean margin sediments. To assess whether this proxy is also applicable to the sedimentary record in tropical lacustrine systems, we investigated the distribution of branched glycerol dialkyl glycerol tetraethers (GDGTs) in recently deposited sediments from 46 lakes in tropical East Africa. These lakes span a substantial range in surface elevation (770-4500 m above sea level), and thus also a wide gradient of mean annual temperature. We find that, saline lakes excepted, branched GDGTs are universally abundant in the lakes investigated and can be used to predict mean annual air temperature (MAAT) with a high degree of accuracy. However, the existing global MBT/CBT calibration for MAAT based on soils predicts inaccurate temperatures when applied to our African lake dataset. This observation, together with the fact that surface water pH, and to lesser extent, lake depth appear to influence the distribution of branched GDGTs among sites, leads us to conclude that in situ production of branched GDGTs in lakes is likely. The robust relationship between branched GDGT distribution and the temperature and pH of African freshwater lakes makes these compounds suitable for paleoenvironmental reconstruction, however we urge caution in using branched GDGTs in lake sediments to infer past temperatures, unless their exact origin can be determined.     

O/O and D/H ratios of pedogenic kaolinite in a North American Cenomanian laterite: Paleoclimatic implications

Kaolinite, gibbsite and quartz are the dominant minerals in samples collected from two outcrops of a Cenomanian (∼95 Ma) laterite in southwestern Minnesota. A combination of measured yields and isotope ratios permitted mass balance calculations of the δD and δ¹⁸O values of the kaolinite in these samples. These calculations yielded kaolinite δD values of about −73‰ and δ¹⁸O values of about +18.7‰. The δD and δ¹⁸O values appear to preserve information on the ancient weathering system.If formed in hydrogen and oxygen isotope equilibrium with water characterized by the global meteoric water line (GMWL), the kaolinite δD and δ¹⁸O values indicate a crystallization temperature of 22 (±5) °C. A nominal paleotemperature of 22 °C implies a δ¹⁸O value for the corresponding water of −6.3‰. The combination of temperature and meteoric water δ¹⁸O values is consistent with relatively intense rainfall at that mid-paleolatitude location (∼40°N) on the eastern shore of the North American Western Interior Seaway. The inferred Cenomanian paleosol temperature of ∼22 °C is in general accord with published mid-Cretaceous continental mean annual temperatures (MAT) estimated from leaf margin analyses of fossil plants.When compared with results from a published GCM-based Cenomanian climate simulation which specifies a latitudinal sea surface temperature (SST) gradient that was either near modern or smaller-than-modern, the kaolinite paleotemperature of 22 °C is closer to the GCM-predicted MAT for a smaller equator-to-pole temperature difference in the mid-Cretaceous. Moreover, the warm, kaolinite-derived, mid-paleolatitude temperature of 22 °C is associated with proxy estimates of high concentrations of atmospheric CO₂ in the Cenomanian. The overall similarity of proxy and model results suggests that the general features of Cenomanian continental climate in that North American locale are probably being revealed.     

Summer air temperature,reconstructions from the last glacial stage based on rodents from the site Taillis-des-Coteaux (Vienne), Western France

The oxygen isotope composition of phosphate from tooth enamel of rodents (δ¹⁸O_p) constitutes a valuable proxy to reconstruct past air temperatures in continental environments. This method has been applied to rodent dental remains from three genera, Arvicola sp., Microtus sp. and Dicrostonyx sp., coming from Taillis-des-Coteaux, Vienne, France. This archaeological site contains an exceptionally preserved sedimentary sequence spanning almost the whole Upper Palaeolithic, including seven stratigraphic layers dated from 35 to 17 cal ka BP. The abundant presence of rodent remains offers the opportunity to quantify the climatic fluctuations coeval of the various stages of human occupation of the site. Differences between δ¹⁸O_p values of Arvicola sp. and Microtus sp. teeth are interpreted as the result of heterochrony in tooth formation as well as differences in ecology. Mean δ¹⁸O_p values of Microtus sp. are preferentially used to reconstruct summer air temperatures, which range from 16.0 ± 3.7 to 19.1 ± 3.1°C throughout the sedimentary sequence; however, the highest variability is observed during the last glacial maximum.     

Contrasting variability in foraminiferal and organic paleotemperature proxies in sedimenting particles of the Mozambique Channel (SW Indian Ocean)

Accurate sea surface temperature (SST) proxies are important for understanding past ocean and climate systems. Here, we examine material collected from a deep-moored sediment trap in the Mozambique Channel (SW Indian Ocean) to constrain and compare both inorganic (δ¹⁸O, Mg/Ca) and organic (, TEX₈₆) temperature proxies in a highly dynamic oceanographic setting for application in paleoceanography. High-resolution time-series current velocity data from long-term moorings (2003 - present) deployed across the Mozambique Channel reveal the periodic migration of four to six meso-scale eddies through the channel per year. These meso-scale eddies strongly influence water mass properties including temperature and salinity. Despite the dynamic oceanographic setting, fluxes of the surface-dwelling planktonic foraminifera Globigerinoidesruber and Globigerinoides trilobus follow a seasonal pattern. Temperatures reconstructed from G. ruber and G. trilobus δ¹⁸O and Mg/Ca closely mirror seasonal SST variability and their flux-weighted annual mean SSTs of 28.1 °C and 27.3 °C are in close agreement with annual mean satellite SST (27.6 °C). The sub-surface dwelling foraminifera Neogloboquadrina dutertrei and Globigerinoides scitula recorded high-frequency temperature variations that, on average, reflect conditions at water depths of 50-70 m and 200-250 m, respectively. Concentrations and fluxes of organic compounds (alkenones and crenarchaeol) display no or only moderate seasonality but flux weighted means of the associated temperature signatures, , and of 28.3 °C and 28.1 °C, respectively, also closely reflect mean annual SST. We analyzed all time-series data using multiple statistical approaches including cross-correlation and spectral analysis. Eddy variability was clearly expressed in the statistical analysis of physical oceanographic parameters (current velocity and sub-surface temperature) and revealed a frequency of four to six cycles per year. In contrast, statistical analysis of proxy data from the sediment trap did not reveal a significant coupling between eddy migration and organic compound fluxes or reconstructed temperatures. This is likely a result of the relatively low resolution (21 days) and short (2.5 years) duration of the time series, which is close to the detection limit of the eddy frequency.     

The effect of Holocene temperature fluctuations on the evolution and ecology of Neotoma (woodrats) in Idaho and northwestern Utah

Animals respond to climatic change by adapting or by altering distributional patterns. How an animal responds is influenced by where it is positioned within its geographic range; the probability of extirpation is increased near range boundaries. Here, we examine the impact of Holocene climatic fluctuations on a small mammalian herbivore, the bushy-tailed woodrat (Neotoma cinerea), at five locations within south central Idaho and northwestern Utah. Previous work demonstrated that woodrats adapt to temperature shifts by altering body size. We focus here on the relationship between body mass, temperature, and location within the geographic range. Body mass is estimated by measuring fossil fecal pellets, a technique validated in earlier work. Overall, we find the predicted phenotypic response to climate change: animals were larger during cold periods, and smaller during warmer episodes. However, we also identify several time periods when changes in environmental temperature exceeded the adaptive flexibility of N. cinerea. A smaller-bodied species, the desert woodrat (N. lepida) apparently invaded lower elevation sites during the mid-Holocene, despite being behaviorally and physically subordinate to N. cinerea. Analysis of contemporary patterns of body size and thermal tolerances for both woodrat species suggests this was because of the greater heat tolerance of N. lepida. The robust spatial relationship between contemporary body size and ambient temperature is used as a proxy to reconstruct local climate during the Holocene.     

Quantifying 21st-century France climate change and related uncertainties

We tackle here the question of past and future climate change at sub-regional or country scale with the example of France. We assess France climate evolution during the 20th and 21st century as simulated by an exhaustive range of global climate simulations. We first show that the large observed warming of the last 30 years can be simulated only if anthropogenic forcings are taken into account. We also suggest that human influence could have made a substantial contribution to the observed 20th century multi-decadal temperature fluctuations. We then show that France averaged annual mean temperature at the end of the 21st century is projected to be on the order of 4.5 K warmer than in the early 20th century under the radiative concentration pathways 8.5 (RCP8.5) scenario. Summer changes are greater than their winter counterpart (6 K versus 3.7 K). Near-future (2020–2049) changes are on the order of 2.1 K (with 2.6 K in summer and 1.8 K in winter). Model projections also suggest a substantial summer precipitation decrease (−0.6 mm/day), in particular over southern France, and a moderate winter increase, (0.3 mm/day), mostly over the northernmost part of France. Uncertainties about the amplitude of these precipitation changes remain large. We then quantify the various sources of uncertainty and study how their ranking varies with time. We also propose a physically-based metric approach to reduce model uncertainty and illustrate it with the case of summer temperature changes. Finally, timing and amplitude of France climate change in case of a global average 2-K warming are investigated. Aggressive mitigation pathways (such as RCP2.6) are absolutely required to avoid crossing or barely exceeding the 2-K global threshold. However, France climate change requiring adaptation measures is still to be expected even if we achieve to remain below the 2-K global target.     

Temperature effects on the calcite skeletal composition of deep-water gorgonians (Isididae)

We test for and calibrate a proxy for ocean temperature based on the skeletal composition of the widely distributed, deep-sea gorgonians in the family Isididae (bamboo corals), through use of three complementary methods: a short-term comparison of element/Ca ratios to a four-year temperature record, a long-term comparison with oceanographic records spanning forty years, and a geographic comparison of Isidids collected at sites ranging from the tropics to Antarctica. The assays consistently support a temperature-dependency for Mg/Ca ratios and suggest S/Ca is indirectly affected by temperature, but indicate little or no effect of temperature on P/Ca and Sr/Ca. The consensus relationship between Mg/Ca and temperature for Isidid calcite from the comparisons with the temperature time-series is T = −0.505 + 0.048 Mg/Ca, where T is in °C, Mg/Ca is in mmol/mol, and the applicable range is 3-6 °C. The results of the geographic assay, though imprecise, suggest the applicable range extends to temperatures below freezing. The scatter of data points around the regression of temperature and Mg/Ca is wide in all assays. This could reflect the effect of factors other than temperature on Mg/Ca ratios, but is also likely to reflect limitations of the field data, the effects of assumed constant growth rates in the corals and instrumental analytical error. The combined effects of micro-scale variability in growth rates and wide confidence intervals for each data point suggests that environmental reconstruction from Isidid internode calcite from sparse data or at time scales less than decades be done with caution. Comparisons within and among colonies do not indicate strong vital effects on ontogenetic variability in the corals, other than possibly close to the central pore of the coral. However, similar Mg/Ca ratios for Isidids from Antarctic and more temperate regions suggest adaptation to local conditions and hence a role for physiology at higher taxonomic levels, at least. Taxonomically higher level vital effects are also suggested by large differences between gorgonian families in their regressions between Mg/Ca and temperature, by Mg/Ca ratios that overlap over a wide temperature and habitat range, and for a non-linear relationship between temperature and the slope of the Mg/Ca-temperature relationship across the order.     

CO2-forced climate thresholds during the Phanerozoic

The correspondence between atmospheric CO₂ concentrations and globally averaged surface temperatures in the recent past suggests that this coupling may be of great antiquity. Here, I compare 490 published proxy records of CO₂ spanning the Ordovician to Neogene with records of global cool events to evaluate the strength of CO₂-temperature coupling over the Phanerozoic (last 542 my). For periods with sufficient CO₂ coverage, all cool events are associated with CO₂ levels below 1000 ppm. A CO₂ threshold of below ∼500 ppm is suggested for the initiation of widespread, continental glaciations, although this threshold was likely higher during the Paleozoic due to a lower solar luminosity at that time. Also, based on data from the Jurassic and Cretaceous, a CO₂ threshold of below ∼1000 ppm is proposed for the initiation of cool non-glacial conditions. A pervasive, tight correlation between CO₂ and temperature is found both at coarse (10 my timescales) and fine resolutions up to the temporal limits of the data set (million-year timescales), indicating that CO₂, operating in combination with many other factors such as solar luminosity and paleogeography, has imparted strong control over global temperatures for much of the Phanerozoic.     

Can sedimentary leaf waxes record D/H ratios of continental precipitation? Field, model, and experimental assessments

D/H ratios of leaf waxes (δD_wax) derived from terrestrial plants and preserved in lake sediments can provide important information on past continental hydrology. Ideally, δD_wax can be used to reconstruct precipitation D/H ratios (δD_P) which is a well-established paleoclimate proxy. However, many other factors, such as vegetation and relative humidity (RH), also affect δD_wax variation. How the combination of these factors affects sedimentary δD_wax is unclear. Here, we use a transect of 32 lake surface sediments across large gradients of precipitation, relative humidity, and vegetation composition in the southwestern United States to study the natural factors affecting sedimentary δD_wax. δD values of C₂₈n-alkanoic acids show significant correlation with δD_P values (R² = 0.76) with an apparent isotopic enrichment of ∼99 ± 8‰, indicating that sedimentary δD_wax values track overall δD_P variation along the entire transect. Leaf waxes produced by plants grown under controlled conditions (RH = 80%, 60%, 40%) show a small increase in D/H ratios as RH decreases, consistent with prediction from the Craig-Gordon model. However, the isotopic effect of RH on δD_wax along the natural transect is partially countered by the opposing influence of vegetation changes. The correlation between δD_wax and δD_P values is significantly higher (R² = 0.84) in the drier portions of the transect than in the wetter regions (R² = 0.64). This study suggests that D/H ratios of sedimentary leaf waxes can be used as a proxy for precipitation δD variations, with particularly high fidelity in dry regions, although more studies in other regions will be important to further test this proxy.     

Coral skeleton P/Ca proxy for seawater phosphate: Multi-colony calibration with a contemporaneous seawater phosphate record

A geochemical proxy for surface ocean nutrient concentrations recorded in coral skeleton could provide new insight into the connections between sub-seasonal to centennial scale nutrient dynamics, ocean physics, and primary production in the past. Previous work showed that coralline P/Ca, a novel seawater phosphate proxy, varies synchronously with annual upwelling-driven cycles in surface water phosphate concentration. However, paired contemporaneous seawater phosphate time-series data, needed for rigorous calibration of the new proxy, were lacking. Here we present further development of the P/Ca proxy in Porites lutea and Montastrea sp. corals, showing that skeletal P/Ca in colonies from geographically distinct oceanic nutrient regimes is a linear function of seawater phosphate (PO_4 SW) concentration. Further, high-resolution P/Ca records in multiple colonies of Pavona gigantea and Porites lobata corals grown at the same upwelling location in the Gulf of Panamá were strongly correlated to a contemporaneous time-series record of surface water PO_4 SW at this site (r² = 0.7-0.9). This study supports application of the following multi-colony calibration equations to down-core records from comparable upwelling sites, resulting in ±0.2 and ±0.1 μmol/kg uncertainties in PO_4 SW reconstructions from P. lobata and P. gigantea, respectively.

     

Anthropogenic versus climatic control in a high-resolution 1500-year chironomid stratigraphy from a southwestern Greenland lake

We performed a high-resolution study of chironomid assemblages in a sediment core retrieved from Lake Igaliku in southern Greenland. The well-dated core is located within the former Norse Eastern Settlement and covered the last 1500 yr. The comparison of chironomid stratigraphy (PCA axis scores) with instrumental temperature data, land use history and organic matter in the sediment over the last 140 yr suggested that the primary changes in chironomid fauna in 1988 ± 2 yr were driven by the shift to modern agriculture in the catchment. This unprecedented change in chironomid fauna was most likely triggered by a shift in in-lake processes. Within the instrumental period, subtle variations in the chironomid assemblages that occurred before 1988 ± 2 yr were significantly correlated with summer temperatures even in times of traditional extensive sheep farming in the catchment. The relevance of the chironomid-derived climate signal over the last 1500 yr was supported by its good concordance with previous studies in west Greenland and in the Arctic. The chironomid assemblage therefore appeared to be a valuable proxy for climate changes within the Norse colony area. Synchronous changes in Norse diet and chironomid-reconstructed climate give new insights into the interplay of Norse society with climate.